internal/app: check username validation

This stuff should be hardcoded in libc, but check it anyway.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/sysconf_test.go

@@ -0,0 +1,15 @@
+package app
+
+import "testing"
+
+const (
+	_POSIX_LOGIN_NAME_MAX = 9
+)
+
+func TestSysconf(t *testing.T) {
+	t.Run("LOGIN_NAME_MAX", func(t *testing.T) {
+		if got := sysconf(_SC_LOGIN_NAME_MAX); got < _POSIX_LOGIN_NAME_MAX {
+			t.Errorf("sysconf(_SC_LOGIN_NAME_MAX): %d < _POSIX_LOGIN_NAME_MAX", got)
+		}
+	})
+}